500 mpa grade low yield ratio weather-resistant bridge steel and manufacturing method therefor

ABSTRACT

Disclosed is 500-MPa low-yield-ratio weather-resistant bridge steel and a manufacturing method therefor; the weather-resistant bridge steel includes the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities; through scientific component designing and a matched manufacturing method combining controlled rolling and cooling and tempering, the weather-resistant bridge steel has a low yield ratio, high low-temperature toughness and high elongation.

TECHNICAL FIELD

The present disclosure relates to weather-resistant steel, and particularly relates to 500-MPa low-yield-ratio weather-resistant bridge steel and a manufacturing method therefor.

BACKGROUND ART

With the long-span, heavy-load and all-welded structure development of large steel structure bridges, safety and reliability of bridge structures are requested more and more strictly. This puts forward higher requirements for designers and higher standards for steel plate quality. That is, a steel plate is required to have not only high strength to satisfy a lightweight structure requirement, but also excellent plasticity, low-temperature toughness, weldability, corrosion resistance, etc. to meet working conditions of a longer bridge span and a heavier load. Therefore, bridge steel, with high strength, high toughness, high plasticity and high weather resistance, can be used to reduce a dead weight of a bridge structure, reduce difficulty of bridge engineering design, manufacture and construction, and prolong the service life.

However, when strength of steel increases, a yield ratio of the steel can generally increase, even reaching 0.93 or more. Because of a high yield ratio, once a component is overloaded, for example, under earthquake or other conditions, steel ultimate strength will be reached rapidly, and then accidents will occur. Thus, the high yield ratio limits use of high-strength structural steel in bridge engineering. The frequent occurrence of earthquakes and their disastrous consequences have aroused great concern abroad about earthquake resistance of bridges, such that relevant provisions have been made in some structural design specifications. Bridge steel with a low yield ratio, high strength, high toughness, high plasticity and high weather resistance is the development trend of bridge construction.

The invention with the publication number CN101892431A discloses hot rolled weatherproof bridge steel with yield strength of a 500 MPa level and a manufacturing method thereof. The steel has a Mo content of 0.15%-0.35% and requires rare earth treatment, so the cost is high. The invention with the application number of CN201811494831.3 provides Q550qENH and a production method therefor, but a thickness of a steel plate is 12 mm-60 mm, and a yield ratio of steel plates with some thicknesses reaches 0.87. The invention with the publication number of CN103103458A discloses high-strength weather-resistant steel and a preparation method therefor. However, with a C content of 0.01%-0.05%, the steel belongs to ultra-low carbon steel and is very difficult to smelt. Further, with a Mn content of 1.5%-2.0%, a casting billet is prone to center segregation. A final product is a thin coil plate, which cannot satisfy requirements of thick and wide modern bridge structure construction.

SUMMARY

An objective of the present disclosure is to overcome defects in the prior art. Disclosed is 500-MPa low-yield-ratio weather-resistant bridge steel. The weather-resistant bridge steel has a low yield ratio, high low-temperature toughness and high elongation.

Another objective of the present disclosure is to provide a manufacturing method for the 500-MPa low-yield-ratio weather-resistant bridge steel.

Technical solution: the 500-MPa low-yield-ratio weather-resistant bridge steel described in the present disclosure includes the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities.

In the present disclosure, determination of a component ratio is achieved by changing contents of some elements and adding alloy elements capable of strengthening and improving mechanical properties of a material. A principle of using multiple elements in small amounts is followed, and a method combining controlled rolling and cooling and tempering is used, such that steel having a thickness of 8 mm-80 mm and a yield ratio ≤0.83 is obtained, with a delivery state after thermal mechanical control processing (TMCP) and tempering.

Specifically, in the components of the present disclosure, a content of carbon in the steel should not be too high, and moreover, sulfur and gas impurities are reduced and a content of a corrosion-resistant element phosphorus is controlled, so as to ensure purity and toughness of the steel. According to the present disclosure, a low yield ratio, high low-temperature toughness and high elongation are achieved by means of a structure type mainly composed of tempered bainite. The components and contents thereof are described as follows:

In the steel, C is an indispensable element for improving strength and hardness of the steel, and has a significant influence on a steel structure. C dissolves into a matrix to form an interstitial solid solution, so as to achieve solution strengthening and significantly increase strength of the matrix. When a carbon content increases, tensile strength and a yield limit of the steel may increase while elongation and notch impact toughness may decrease. When a C content in the steel is high, generation of cold cracks may be aggravated. Therefore, the present disclosure uses ultra-low carbon design, and a small amount of C forms microalloyed element carbide in the steel, so as to achieve second phase strengthening and grain refining. A percentage of C in the present disclosure is set to 0.04%-0.09%.

Mn is a main element in railway standard steel, and is capable of improving strength of a material. Although a C or Cr content can also be increased to improve the strength, too much carbon influences formability and a weld line, while Cr is too high in price but limited in reserves, which is not conducive to cost reduction. Mn is also a main element for preventing hot shortness in the steel, so a manganese content is increased to an upper limit after comprehensive consideration. A percentage of Mn in the present disclosure is set to 1.40%-1.50%.

In the steel, Si improves the strength of the steel mainly through strong solution strengthening, and is also a necessary element for deoxidization in steelmaking. The element is capable of improving atmospheric corrosion resistance, but may obviously reduce plasticity, toughness and a surface coating property of the steel. Therefore, considering the strength, toughness, plasticity and other factors comprehensively, a percentage of Si in the present disclosure is set to 0.15%-0.30%.

P promotes amorphous transition of a rust layer. Generally speaking, a Cu-P composite has an optimal weather resistance effect and is a relatively economical corrosion-resistant element. Considering that P leads to cold shortness and crack sensitivity, a P content is generally limited in weather-resistant steel for important welded structures. A narrow range of a percentage of P in the present disclosure is controlled to be 0.009%-0.012%.

Cu mainly plays a role of solution strengthening in the steel. A proper amount of copper is capable of improving the strength without reducing the toughness, and is also capable of enhancing corrosion resistance of the steel. A percentage of Cu in the present disclosure is 0.30%-0.40%.

Cr is widely used in practical industrial production, is second only to carbon in improving yield strength of the steel, and is not conducive to yield ratio reduction. In addition, China has less chromium reserves, so a chromium content is reduced, and Mn and Si are used instead. A percentage of Cr in the present disclosure is 0.45%-0.60%.

Mo is a strong solution strengthening element, may strongly improve hardenability, may greatly improve red hardness, may improve tempering stability and obviously reduce tempering brittleness. A percentage of Mo in the present disclosure is 0.08%-0.15%.

V is a moderate carbide forming element, is capable of forming alloy carbide VC having a simple cubic crystal structure, and is capable of entering cementite, to improve cementite stability and the tempering stability. A percentage of V in the present disclosure is 0.010%-0.030%.

Ti is capable of enabling a C curve to move rightwards, may obviously improve the strength and achieve grain refining, and may further improve the toughness of the steel. A proper amount of Ti may form a second material particle and improve the toughness of the metal. A percentage of Ti in the present disclosure is 0.010%-0.020%.

Furthermore, an atmospheric corrosion resistance index I of the steel ≥ 6.5. The atmospheric corrosion resistance index I=26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P)-7.29(%Cu)(%Ni)-9.10(%Ni)(%P)-33.39(%Cu)².

For the 500 MPa low-yield-ratio weather-resistant bridge steel, the technical solution used by the manufacturing method provided in the present disclosure includes processes of smelting, continuous casting, soaking, rolling, relaxation, cooling and off-line tempering.

A continuous casting billet is heated in the soaking process until a center temperature reaches 1130° C.-1230° C.

The rolling process is to conduct recrystallization zone rolling and non-recrystallization zone rolling on a descaled continuous casting billet, and an accumulated deformation amount of the recrystallization zone rolling is 50% or more of a thickness of the continuous casting billet.

An intermediate billet holds a temperature at 800° C.-990° C., a temperature-holding thickness is 2 times-4 times of a final-product thickness, the non-recrystallization zone rolling is conducted after a temperature is reached, and a finishing temperature is controlled to be 790° C.-830° C.

In the relaxation process, relaxation is conducted until an initial cooling temperature is 730° C.-760° C.

The cooling process is to conduct laminar cooling from the initial cooling temperature, control a self-tempering temperature to be 420° C.-600° C., and then conduct air-cooling to a room temperature.

In the off-line tempering process, a tempering temperature is 450° C.-550° C., heat preservation is conducted at the temperature for 20 min-40 min, heat preservation time being proportionate to the final-product thickness, and then natural cooling is conducted to the room temperature.

Beneficial effects: compared with the prior art, in the present disclosure, through scientific component designing and a matched manufacturing method combining controlled rolling and cooling and tempering, the 500-MPa low-yield-ratio weather-resistant bridge steel having a low yield ratio, high toughness and high elongation is obtained. The steel has the yield strength of 545 MPa or more and the tensile strength of 682 MPa or more, finished steel has a yield ratio less than or equal to 0.83, -40° C. Akv of 180 J or more and elongation ≥20%, and the steel has desirable comprehensive properties and is suitable for use in bridge structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical metallographic structure image of a product after magnification of 500 times in Embodiment 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure is further described below with reference to specific embodiments.

Embodiment 1 included the components in percentage by mass: C: 0.04%, Si: 0.28%, Mn: 1.50%, P: 0.014%, S: 0.0010%, Nb: 0.020%, Ti: 0.015%, V: 0.010%, Ni: 0.30%, Cu: 0.40%, Cr: 0.45%, Mo: 0.08%, Alt: 0.02%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 150 mm and an atmospheric corrosion resistance index I being 6.51. Stacking cooling was conducted on a continuous casting billet 24 h or more, and the billet was soaked at 1230° C., a uniform temperature being less than 20° C. After heating was conducted for 150 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1080° C., a total deformation amount was 79%, and a thickness of an intermediate billet was controlled to be 4 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 990° C., a thickness of the final product was 8 mm, and a finishing temperature was 830° C.

After rolling finishing, relaxation was conducted to an initial cooling temperature of 730° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 600° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 550° C., and heat preservation was conducted at the temperature for 20 min.

Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 575 MPa, tensile strength of 693 MPa, a yield ratio of finished steel of 0.83, -40° C. Akv of 180 J, and elongation A of 20%.

Embodiment 2 included the components in percentage by mass: C: 0.06%, Si: 0.30%, Mn: 1.46%, P: 0.010%, S: 0.0015%, Nb: 0.040%, Ti: 0.020%, V: 0.020%, Ni: 0.35%, Cu: 0.30%, Cr: 0.60%, Mo: 0.10%, Alt: 0.04%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 320 mm and an atmospheric corrosion resistance index I being 6.70. Stacking cooling was conducted on a continuous casting billet 48 h or more, and the billet was soaked at 1160° C., a uniform temperature being less than 20° C. After heating was conducted for 352 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1070° C., a total deformation amount was 53%, and a thickness of an intermediate billet was controlled to be 2.5 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 850° C., a thickness of the final product was 60 mm, and a finishing temperature was 810° C.

After rolling finishing, relaxation was conducted to an initial cooling temperature of 750° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 480° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 500° C., and heat preservation was conducted at the temperature for 35 min.

As shown in FIG. 1 , through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 556 MPa, tensile strength of 682 MPa, a yield ratio of finished steel of 0.82, -40° C. Akv of 225 J, and elongation A of 21%.

Embodiment 3 included the components in percentage by mass: C: 0.09%, Si: 0.15%, Mn: 1.40%, P: 0.0090%, S: 0.0020%, Nb: 0.035%, Ti: 0.018%, V: 0.030%, Ni: 0.45%, Cu: 0.37%, Cr: 0.50%, Mo: 0.15%, Alt: 0.02%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 320 mm and an atmospheric corrosion resistance index I being 6.53. Stacking cooling was conducted on the casting billet 48 h or more, and the billet was soaked at 1130° C., a uniform temperature being less than 20° C. After heating was conducted for 320 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone rolling was 1040° C., a total deformation amount of rough rolling was 50%, and a thickness of an intermediate billet was controlled to be 2.0 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 800° C., a thickness of the final product was 80 mm, and a finishing temperature was 790° C.

After rolling finishing, relaxation was conducted to an initial cooling temperature of 760° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 420° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 450° C., and heat preservation was conducted at the temperature for 40 min.

Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type at low magnification was “tempered bainite” with high uniformity, and the material had yield strength of 545 MPa, tensile strength of 673 MPa, a yield ratio of finished steel of 0.81, -40° C. Akv of 216 J, and elongation A of 22%.

Embodiment 4 included the components in percentage by mass: C: 0.05%, Si: 0.20%, Mn: 1.45%, P: 0.015%, S: 0.0012%, Nb: 0.050%, Ti: 0.010%, V: 0.018%, Ni: 0.40%, Cu: 0.38%, Cr: 0.48%, Mo: 0.12%, Alt: 0.025%, and the balance Fe and inevitable impurities. Smelting, refining, alloying and calcium treatment were conducted on raw materials, then molten steel was obtained, and the molten steel was subjected to slab continuous casting, a thickness of a casting billet being 260 mm and an atmospheric corrosion resistance index I being 6.59. Stacking cooling was conducted on the casting billet 36 h or more, and the billet was soaked at 1200° C., a uniform temperature being less than 20° C. After heating was conducted for 286 min, dephosphorization was conducted, and then rolling was conducted at two stages. A temperature of recrystallization zone finish rolling was 1100° C., a total deformation amount of rough rolling was 63%, and a thickness of an intermediate billet was controlled to be 3.0 times of that of a final product. An initial rolling temperature of non-recrystallization zone rolling was 870° C., a thickness of the final product was 32 mm, and a finishing temperature was 810° C.

After rolling finishing, relaxation was conducted to an initial cooling temperature of 740° C., laminar cooling was conducted on a steel plate at the initial cooling temperature, a self-tempering temperature being 550° C., then air-cooling was conducted to a room temperature, then the steel plate was tempered at a tempering temperature of 480° C., and heat preservation was conducted at the temperature for 30 min.

Through observation on a metallographic structure of a sample after controlled rolling and cooling and tempering, it was found that a microstructure type was “tempered bainite”, and the material had yield strength of 571 MPa, tensile strength of 713 MPa, a yield ratio of finished steel of 0.80, -40° C. Akv of 332 J, and elongation A of 21%.

It may be seen from the above embodiments that for the 500 MPa low-yield-ratio weather-resistant bridge steel produced by a heavy and medium plate mill, the yield ratio of the weather-resistant bridge steel is effectively reduced through component designing and a matched manufacturing process including controlled rolling and cooling and off-line tempering, and a yield ratio of finished steel may be ensured to be ≤0.83. 

What is claimed is:
 1. 500 MPa low-yield-ratio weather-resistant bridge steel, comprising the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities.
 2. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein a metallographic structure is tempered bainite.
 3. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein a 8 mm-80 mm thick steel plate has a yield ratio ≤0.83.
 4. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 3, wherein an atmospheric corrosion resistance index I ≥6.5.
 5. The 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, wherein in the components in percentage by mass, C is 0.06%-0.09% and Mn is 1.40%-1.46%.
 6. A manufacturing method for the 500-MPa low-yield-ratio weather-resistant bridge steel according to claim 1, comprising processes of smelting, continuous casting, soaking, rolling, relaxation, cooling and off-line tempering, wherein a continuous casting billet is heated in the soaking process until a center temperature reaches 1130° C.-1230° C.; the rolling process is to conduct recrystallization zone rolling and non-recrystallization zone rolling on a descaled continuous casting billet, and an accumulated deformation amount of the recrystallization zone rolling is 50% or more of a thickness of the continuous casting billet; an intermediate billet holds a temperature at 800° C.-990° C., a temperature-holding thickness is 2 times-4 times of a final-product thickness, the non-recrystallization zone rolling is conducted after a temperature is reached, and a finishing temperature is controlled to be 790° C.-830° C.; in the relaxation process, relaxation is conducted until an initial cooling temperature is 730° C.-760° C.; the cooling process is to conduct laminar cooling from the initial cooling temperature, control a self-tempering temperature to be 420° C.-600° C., and then conduct air-cooling to a room temperature; and in the off-line tempering process, a tempering temperature is 450° C.-550° C., heat preservation is conducted at the temperature for 20 min-40 min, heat preservation time being proportionate to the final-product thickness, and then natural cooling is conducted to the room temperature.
 7. The manufacturing method according to claim 6, wherein a 150 mm-320 mm thick continuous casting billet is used in manufacturing of a 8 mm-80 mm thick final product.
 8. The manufacturing method according to claim 7, wherein in the continuous casting process, stacking cooling is conducted on the continuous casting billet for 24 h or more, stacking cooling time increases with increasing of a thickness of the continuous casting billet, and for a 320 mm continuous casting billet, the stacking cooling time is 48 h or more.
 9. The manufacturing method according to claim 8, wherein a uniform temperature of the continuous casting billet in the soaking process is less than 20° C.
 10. The manufacturing method according to claim 9, wherein in the soaking process, heating time ≥ the thickness of the continuous casting billet * 1 min/mm. 